Variable frequency relaxation oscillator



M. E. MOHR VARIABLE FREQUENCY RELAXATION OSCILLATOR April 15, 1952 4 Sheets-Sheet l Filed June 2, 1949 kb@ QESM.

/NVENTOR ME MOH/i BV @f/QQQM% ATTORNEY Apm 15, 1952 Filed June 2, 1949 M. E. MoHR 2,593,330

VARIABLE FREQUENCY RELAXATION oscILLAToR 4 Sheets-Sheet 2 April l5, 1952 M E, MOHR VARIABLE FREQUENCY RELAXATION OSCILLATOR 4 Sheets-Sheet 5 Filed June 2, 1949 kblks /NVE/VTOR M E. MOH/P TR/V V April 15.l 1952 M E MQHR VARIABLE FREQUENCY RELAXATION OSCILLATOR 4 Sheets-Sheet 4 Filed June 2, 1949 www QN 5) @Q QQ f v2 NE /NVNTOR MEMO/ff? BV.'

A TTORNEV Patented Apr. 15, 1952 VARIABLE FREQUENCY RELAXATION OSCILLATOR Milton E. Mohr, New Providence, N. J., Iassignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 2, 1949, Serial No. 96,706

(Cl. Z50- 36) 9 Claims.

This invention relates to electrical systems and more particularly to oscillatory systems in which the frequency of oscillation may be changed between predetermined limits.

In the electric signaling art, it is frequently desirable to determine the frequency composition of a complex signal wave, such as a speech signal wave. In such waves the instantaneous amplitude may rapidly vary in a complex manner dependent upon the instantaneous phase relations of a plurality of different wave components, each of which represents a different sound in the original signal; or a different frequency, or narrow band of frequencies, in the electrical equivalent of the original signal. The determined frequency composition may be utilized in a number of ways, one of which is to depict graphically the varying composition of the Wave in spectographic form, such that the dimensional coordinates of the graph represented frequency and time, respectively, and the degree of brightness of each coordinate point in a visual graphic representation indicates the mean intensity of a particular frequency component at a particular instant of time. A system in which a complex signal wave may be dissected and graphically displayed in the abovedescribed manner is disclosed in United States Patent 2,403,986, dated July 16, 1946, to Lester Y. Lacy, reference to which is hereby made.

In systems of this type a complex signal wave is modulated with the output of an oscillator, the frequency of which is swept, or varied, between predetermined limits, and one of the sideband products of modulation is applied to the input of a relatively narrow band-pass filter. The frequency of the oscillators is varied such that desired sideband is repeatedly moved, or swept, across the pass-band frequency of the filter. The effect of this action is that the lter virtually repetitively traverses the sideband modulation component, and during each traverse selectively passes, or segregates, the various frequency com- -V ponents of the sideband.

It is an object of the present invention to improve the means for segregating the components of a complex wave by providing an improved variable frequency modulating wave.

v It is also an object of this invention to improve the oscillatory characteristics of relaxation type oscillators.

In an embodiment of the invention that is to usual stability with linearity of variation as the control parameters are varied. This wave is obl tained from a unique and novel relaxation oscillator of the multivibrator type, the frequency of which may be varied between predetermined maximum and minimum values in substantially equal discrete steps. Control of the changes in the oscillators frequency resides in a stair-step voltage wave generator which utilizes the socalled bucket and dipper. method of charging a storage, or Sweep, capacitor, and in which the capacitor is discharged in an exceptionally short interval, thereby realizing an unusually short restoration, or iiy-back time. A second control arrangement is provided for the oscillator byv means of which the oscillating frequency, at one of its limiting conditions, is checked after each frequency sweep, and corrective adjustments are applied lwhen the measured frequency differs from a predetermined value.

The nature of the invention and its various features, objects and advantages will be more fully understood from a consideration of the following description of one embodiment, illustrated in the accompanying drawing in which:

Fig. l illustrates diagrammatically an arrangement wherein the oscillatory system of the invention may be incorporated as a part of a wave analyzing system;

Figs. 2, 3 and 4 are schematic diagrams of the stair-step wave generator, the relaxation oscillator and the frequency stabilizing circuits, respectively, in accordance with the invention, and incorporated in the arrangement of Fig. 1; and

Fig. 5 indicates the manner in which Figs. 2, 3 and 4 should be arranged to show the complete schematic diagram of the described embodiment.

y General description `Referring now to Fig. l, there is shown diagrammatically a portion cf a wave analyzing system incorporating the invention. The arrangement of Fig. l comprises a source I0 of signal waves, a modulator I2, an oscillator I4, a source I5 of trains of impulses or pulses of differing durations and differing recurrence rates, a so-called stair-step or staircase-type wave generator I8, a frequency stabilizing circuit 20 including a frequency-checking branch and a frequencycon trolling branch, and a band-pass filter 22. Such an `arrangement might, with suitable interconnecting arrangements, be incorporated in the visible-speech portraying arrangement disclosed in the above-mentioned Lacy patent.

More particularly, complex signal waves from source I0 are supplied to a balanced modulator I2, which may be any suitable known type,

along with square wave oscillations from oscillator I4, the frequency of which is. controlled by generator I8. Two trains of voltage pulses are supplied by pulse source I6. A first train of these, composed of pulses of relatively short duration having any suitable recurrence rate, for example, 488 a second, is supplied to the generator I8 to cause it to produce an output wave which successively steps, or increases in substantially equal valued increments, between predetermined minimum and maximum` values as such pulses are received. The pulses of the second train of pulses are of relatively long duration, and have a recurrence rate which. varies. as the reciprocal of the number of step increments that are to be included in the outputwave of generator I8. These relatively long pulses are delivered simultaneously to the generator I8 and to the'frequency checking branch of the frequencyy stabilizing circuit 28. The stepping operation ofl generator I8 is suspended'during the interval of each long pulse of this second train of pulses, andthe generator is reset, or reconditioned, to start the production of a new stepped wave at the end of the pulsey period. Simultaneously, oscillator I4 is restored to its original operating conditions, and its output frequency is observed by the frequency-checking branch of stabilizing circuit 20. This frequency Y observation or determination is made by observing the voltage that is developed in a frequencysensitive circuit by oscillations from oscillator I4,y and any discrepancy from a predetermined andl desired frequency is transformed into a biasing adjustment that, acting through the fre- 4 quency-control branchv of the stabilizing circuit 20, tends to restore oscillator I4 to its optimum unstepped, or minimum, frequency value. Modulator I2 functions in a conventional manner to produce modulation products from the combined waves. These products are applied to the input of 'a band-pass lter 22 suitably proportioned in known manner, the pass-band and center frequency of which are suitably correlated with the. frequencies of the signal wave from sourceY I and the modulating wave from oscillator I4. The output from filter 22 may be appliedin any desired manner, for example, to control theintensity of a cathode-ray beam as is done in the aforementioned Lacy patent.

Detailed description A specific structure or circuit arrangement for producing a square wave output, the frequency of which is variable in a series of discrete steps between fixed minimum and maximum values, is shown in Figs. 2, 3 and 4, when arranged in the manner shown in Fig. 5.

Referring to Fig. 2, there is shown the stairstep wave generator I8. The circuit of generator I8 comprises a plurality of multiple-section electronVV discharge devices, specifically, the double-triode vacuum tubes 24, 50, 14, 94 and the double-diode vacuum tube 65, and associated resistors and capacitors. The control grid electrode 25 of tube 24 is normally biased positively with respect to its cathode 35, and is connected to pulse input terminal 28 at which the short, sharp, negative' voltage impulses 30 are received from pulse source I6 (Fig. l).Y The left half of tor 88 to pulse input terminal 85, at which the relatively long, widely spaced, positive voltage impulses 3| of the second series of pulses from source I6 (Fig. 1) are received. Resistors 42, 44 form a high-precision voltage divider, to which are connected the anodes of tube 24 and the control electrode 48 of tube 58. Thus, the potential of control electrode 48 is controlled by current conduction in either half of tube 24. Resistors 54, 55, and potentiometer 56 form a voltage divider circuit to whichV the control electrode 52 of tube 55 is connected. When the left half of tube 24 is in'its saturated anode current condition, the potential on control electrode 52 is more positive than is the potential on control electrode 48, ,and the poential of cathode 5l of tube 50 is determined solely by the potential of control electrode 52. Resistor 58 is a cathode load resistor across which is generated a voltage depending upon the current conduction in tube 50. The upper end of resistor 58 is connected over conductor 5I Vto one terminal of the dipper or transfer capacitor 62. The other termi'- nal of capacitor 52. is connected over conductor 53, through the4 left half of double diode 66, that is, anode 61 and' cathode 68, and over switch 55 to the sweep, bucket, or storage capacitor 64. Capacitor 64 is shown as a multisection unit of different values, any one of which may be chosen` by varying the position of switch. S5. The' values of the sections of capacitor 64 are chosen with regard to the size of the voltage steps, or increments, that are desired in the stepped output wave from generator I8. Although an. arrangement for so doing is not specifically shown in the drawing, since it does not. form a part. of this invention, it should be understood that the pulse recurrence rate ofthe previously described relatively long voltage impulses V3:1, of the second train of pulses obtained from pulse source I6 (Fig. l) is Variable,

and" that it is coordinated with the position of switch` 65 such that as the period between these pulses'is made greater or smaller the capacity of capacitor 64 is increased or decreased, proportionately. Thus, the proper coordination of the position of switch 65 and the repetition rate of' the long pulses 3|, from source I 6, results in the production of an output, or sweep, voltage that varies between the same minimum and maximuml values independently ofv the number of steps.' or incremental changes in it.

The upper or ungrounded terminal of capacitor 64 is: connected through switch 65 and over conductor 59 to control electrode 'I0 and anode 'I2Y ofv the double triode 14. The left half of double triode 14. forms a cathode follower, with cathode 'II connected through cathode load resistor 'I6 to a source of negative potential. The upper terminal of resistor 16 is connected over conductor 11 to terminal '18, where it is available for connection to any suitable utilization circuit, such asthe horizontal deflection plates of a cathode-ray tube (not shown), and is also connected over conductor 'Il' to the bias producing combination comprising capacitor 88 and resistor 82, and through resistor 8| to anode 84 ofthe right half of diode 65. The cathode B9 associated with anode 84 is connected to anode 51, and over. conductor 63 to capacitor 52. The bias producing means 80, 82 is provided to prevent undesired charging of capacitor 64 during the interval bel tween pulses at terminal 28. This charging would otherwise occursince the cathode 'II is normally at a higher potential than its control electrode 10, and charging current can fiow over the circuitcomprising cathode 1I, conductor 11, right and left halves of double diode 66, conductor 59, switch 65 and capacitor 64. The potential on capacitor 64is reflected on control electrode and, therefore, unless a blocking bias, such as is developed across the combination 80, 82, is provided, this charging action continues until the left half of double triode 14 becomes overloaded.

Pulses 3| from terminal 86 are applied over conductor 88 and through resistor 90 to control electrode 92 of the tube 94. Control electrode 92 is normally held negative with respect to its associated cathode 93 by the negative potential that it derives through resistors 91, 90. The potential of cathode 93 isv normally somewhat positive with respect to that on control electrode 92 because of its connection to the voltage divider comprising resistors 98, 99. The right half of tube 94 is normally held at anode current saturation by the potential its control electrode |00 derives from the voltage divider comprising resistors |02, |04. Anode 9| is connected to electrode |00 through resistor |02. Anode 95 is connected directly to the control electrode 13 of the right half of tube 14. Anode resistor |05 is of such value that when the right half of tube 94 is at anode current saturation, control electrode 13 is held sufliciently negative with respect to the potential existing on its associated cathode 15 so as to render the right half of tube 14 non-conducting.

A"Referring now to Fig. 3, the relaxation oscillator I4 comprises a plurality of electron discharge devices or vacuum tubes, specifically, triode ||0, tetrodes |06, |08, pentodes |35, |31, |54, unidirectonally conducting devices |22, |24, key |62, and associated resistors and capacitors. Tubes |06, 08 may be operated with a relatively low anode and screen grid voltage supply, which in one tested embodiment was about 60 volts, as obtained from voltage regulating tube |I0. Anode I of triode |0 is connected to a suitable source of positive potential. Control electrode I2 is connected to the movable contact of a potentiometer I3, which forms a portion of the voltage divider circuit comprising resistors |I4, I I6. The potential at electrode I2 controls the magnitude ofthe voltage supplied to anodes ||8, and screen electrodes I|9, |2| of oscillator tubes |06, |08. The cathode electrodes of these latter tubes are connected directly to ground, and a suitable unidirectional conducting device |22, |24,` such as a germanium varistor, is shunted between the control electrode and the cathode of each tube. Anode load resistors |25, |26 are of relatively low value in order to minimize the effects of tube capacity, and in order to obtain desired saturation characteristics in the tubes |06, |08. In one tested embodiment of the invention, in which type 807 oscillator tubes were employed, and in which the oscillating frequency was linearly varied from about 1.22 to 2.0 megacycles per second, it was found desirable to use high-quality deposited-carbon type resistors in these anode circuits instead of the more conventional wirewound units, in order to reduce or eliminate the effects of vresidual inductance. Control grid electrodes |28, |30 are connected to grid return circuits comprising resistor |3| and the upper 6 includes in its circuit load resistor |39, and is also coupled through capacitor |4| to control electrode |30. This cathode is also connected to one terminal of coupling capacitor |40, the other terminal of which is connected over conductor |42 to control electrode |1| of amplifier tube I 44 (Fig. 4). Returning to Fig. 3, cathode: |46 A1of tube |31 includes in its circuit load resistors |41 and |48, and is also coupled through capacitor |43 to control electrode |28 of tube |06. One ter-l minal of output coupling capacitor |50 is connected to the cathode end of resistor |48. The

other terminal of this capacitor may be connected over conductor I5I to a suitable utilization circuit, such as the balanced modulator I2 of Fig. l. The mid-point of potentiometer |33 in the control grid circuits of tubes |06, |08 is connected directly to Vthe cathode electrode |52 of cathode follower tube |54. The controlv electrode |56 of this tube is connected through a decoupling resistor to a suitable potential point on the voltage divider comprising resistors |58, |60, proportioned such that, together with the other voltages that are derived through resistors |61, |68 relaxation oscillator I4 will be biased to a suitable minimum frequency operating value. The common junction point of resistors |56, |60 is also connected to starting key |62, to one contact of which negative potential is connected through resistor |63. Also connected to this common junction point is a voltage dividing resistance network comprising potentiometer |64, resistor |66, and the resistors |61 and |618. ',Ihe upper terminal of potentiometer |64 is connected over conductor 11 to the cathode 1| of tube 14 (Fig. 2), from which a voltage that is representa- `Ative of the charge on capacitor 64 is obtained.

The lower terminal of resistor |68 is connected over conductor |10 and contact I of key |8| to the cathode end of load resistor |15 in the frequency control branch of the frequency stabilizing circuit 20 (Figure 4). The function of the resistance network comprising resistors |58, |60, |66, |61, |68, and potentiometer |64 is to provide a suitable unidirectional potential operating point for the relaxation oscillator, to properly proportion on control electrode |56 the stepped voltage as it is derived from conductor 11, and to provide means for injecting over conductor |10 a frequency controlling voltage derived from the frequency stabilizing circuit 20.

The frequency stabilizing circuit 20, shown in Fig. 4, comprises a voltage feedback circuit including electron discharge devices, specifically, Vacuum tubes |44, |12 and |14, and a delay circuit comprising a pair of single-trip multivibrator circuits including electron discharge devices, specifically, vacuum tubes |16, |18. The control electrode 1| of vacuum tube |44 is connected over conductor |42 and coupling capacitor |40 to cathode |38 of cathode follower |34 (Fig. 3). Vacuum tube |44 forms part of a pentode amplifier. the cathode of which is connected to negative potential through resistor |13. Anode I 11 of tube |44 is connected to a tuned circuit comprising inductor |19 and capacitorA |80, the Values of which are chosen such that the combination resonates at a frequency that is slightly less than the minimum operating frequency of oscillator half of potentiometer |33 and resistor |32 and the lower half of potentiometer |33, respectively The anodes ||8, |20 are connected directly to controlelectrodes |34, |36 of cathode followers |35, |31, respectively. Cathode |38 of tube |35 |4. The operation Q, or quality, of this `combination is suitably about 40, and thus the voltage appearing on anode |11 is a reasonably good sine wave, notwithstanding that the excitation voltage on control electrode |1I is a highly distorted wave of the square type. Anode |11 is coupled throughl capacitor |82 tocontrol electrode; |83 of theleft half of tube |12, the operating bias ofwhich is derivedA over the. movable contact of potentiometer |84, and which may be chosen such that cathode |81 of tube|12 isat a suitable positivepotenti'al with respect to ground when vrelaxation oscillator |4 is operating at its minimum` frequency value. In one tested embodiment of the invention in which the minimum fre,.

stant-thati is relatively long with respect to the oscillations of oscillator I4, for example, 100

microseconds, Therefore, when control electrode |83 is excited byfa'. voltage wavefrom oscillator 4, the potential of cathode |81 increasesto a voltage that corresponds very nearly to the peak value. of the voltage appearing on this control electrode; The` righthalf of tube |12, including control electrode; |90, operates as a cathode follower to repeat across cathode load resistor |92 substantially the same-potential that exists at cathode |81. Theupper terminal' of cathode resistor |92 is connected over conductor |93, the contacts of relay |94, when operated, and through resistor |99 to the storage. capacitor |98 and also tothe control` electrode |91 of cathode follower tube |14. Resistor |99 and capacitor |90 are.V chosen of such values that their time constant is of suicient length to eliminate any tendency toward hunting about the optimum minimum frequency by oscillator |4. The cath.- ode of tube |14 is connected through contact` I of key 8| and conductor |10 tor the lower terv minal of resistor |68, in the control electrode circuit of tube |54 (Fig. 3). Key |8I provides,in the manner indicated, a ready means of initially adjusting the control potentials for suitable operation ofv oscillator I4 and frequency stabilizing circuit 20. The delay circuit comprising double triode. |16 is' connected as a single-trip multi vibrator, in which the control electrode 200 ofA the' right half'of tube |10 is connected to anode potential through resistors 202, 204. Cathodes 205, 206 are connected to the upper terminal of cathode resistor 209. In the absence of any signal on control electrode 2|0, of the left half of Ytube- |16 that half of the multivibrator oscillator is non-conducting. Terminal 86 (Fig. 1) is connected overA conductor 88 and through coupling capacitor 2I4 to control electrode 2|0. The anode 2||v of the left halfof tube |16 is coupled through capacitorL 2|8' to, control electrode 200, andis also coupled through coupling capacitor jectives of the various circuit: portionsa're.- firstthe following description if the functional 011-- recalled. The step-Wave generator I8 (Fig, 2)-L utilizes the so-called dipper" andA bucket method. of charging storage capacitor 64 to produce acrossthe capacitor a potential which changes, in any desired number of substantially equal increments,

between predetermined minimum and' maximum values.

terval, theduration of which is coordinated with the period between-the pulses 3|.l of. the second series of pulses.v At the end. of" each potential sweep, or cycle of increment changes', it isa function of this generator circuitl to; dischargey the storage capacitor to the same minimum.po tential value, and' to perform this discha-rgeffunc' tion in the shortest possible time.`

The relaxation oscillator I4 (Fig. 3) operates-to change linearly its oscillation frequency-'between predetermined minimum and maximum values in accordance with the changes of potentialacross the storage capacitor, orA bucket, ofk the step-wave generator I8. Oscillator I 4varies itsL frequency from the same minimum value tothe same maximum value, regardless of the. number of steps or potential increments that occur during the sweep interval. It mayalso be noted; that the instantaneousfrequency of oscillator` I4 is a-flxed and stable value during the:A period of any potential value of the output wave produced' by generator i 8. The output of this 'oscillator' is suitable for combination with a secondV train of complex waves, such as speech signals, ina utilization circuit, such as a balanced modulator |2 (Fig. l).

It is the function. of the frequency stabiliza` tion circuit 20- (Fig. 4) to-constantlymonitor thev output of oscillator i4, and to derive during each of its periods of minimum frequency avoltage in-- 22-0to5control electrode 2122y of a second delay circuitconsisting' of the single-trip multivibrator comprising the double triode |18. This second multivibrator may besimilarv in all respects to the previouslyV described unit including double triode |16, except that the-equivalentv of resistor 202 is not-included in its control grid circuit.

Thisresistor is included in the circuit of control Operation high negative potential that is derived byits con-I Thiezmannerin; which the-circuitof Figs. 2, 3V

arid/1i;-V operatesmayA bebest;- appreciated. fromV dication of that frequency. This voltage indica-- tion is supplied to the input of the oscillator |4 in a manner that tends to control the oscillator at the predetermined optimum value during, theV next succeeding sweep interval. Because the fre-- quency of oscillator I4 is a varying quantity, itis.

necessary that the frequency stabilizing circuit 20 perform its functions within the. comparatively short interval during which oscillator I4 isoperated at its minimum value. To this end, a timing circuit-is includedfor interconnecting the frequency -checking and frequency ly short interval during each sweep cycle of the oscillator.

Returning, now to Fig. 2, therpulse inverting left half of tube 24 is normally conducting., and its right half is non-conducting becauseof the trol electrode 31 through resistors 39, 49. WhenY no negative pulse 30 exists at terminal 28, thel potential on control electrode 48 is lower than that on electrode 52, and the potentialof. cathodev 51 of tube 50 is determined by the potential that electrode 52 derives from potentiometer. 58,., This.

value fixes the minimum value of the voltage across resistoi' 58. When a negative, voltage im:-v pulse 39 is receivedat terminal 28, it is inverted in thev anode circuit. of the lefthal'fl of tube:- 2.4.,` and momentarily increases the potential of; elec-A trode 48 to a value determinedV by the voltagel divider 42, 44 such that it exceeds the steady` biasing potential on electrode 52. Therefora; the potential of cathode 51 varies between a pre,` determined minimumvalue, asA determined l by;

This range of changing'potential.values is produced once during each predeterminedinn-` control, branches of the stabilizing circuit for a relative-l the potential derived from potentiometer 56, and

apredetermined maximum value as determined by the potential derived from the voltage divider 42, 44. When cathode 51 changes to its maximum potential value, the potential across ycapacitor 62 is changed by an amount corresponding to the difference between these values, which change causes charge to iiow through capacitor 62, through the left side of double diode 66 comprising anode 61 and cathode 68, and into the sweep capacitor 64, connected between cathode 68 and ground. The amount of this charge, or the voltage division across the two capacitors 62, 64 will be proportional to the ratio of the capacities of one of the units to the combined capacities of the two units; and so long as the voltage excursion of cathode 51 is rigidly maintained, the step size, or increment of potential change across capacitor 64 will be a constant value. The potential across capacitor 64 is reproduced, at a slightly more positive value, across cathode resistor 16, and on connecting circuit 11. Thus, when the voltage of capacitor 64 changes, the sweep voltage potential on conductor 11, which is connected to terminal 18 and to the upper terminal of potentiometer |64 (Fig. 3), is also changed.

At the termination of the negative voltage impulse on terminal 28, the potential of the cathode 51 of tube 50 returns to its minimum value, and capacitor 62 discharges over a path comprising resistors 58, 16, conductor 11, resistors 8|, 82, the right half of tube 66, comprising anode 84 and cathode 69, and conductor 63. Because the potential of capacitor 64 is repeated, at a slightly more positive value at the cathode 1| of tube 14, capacitor 62 discharges in such manner that its right terminal acquires substantially the same potential as exists across capacitor 64. The left terminal of capacitor 62 returns to the minimum value potential of cathode 51 of tube 56. Therefore, when cathode 51 again assumes its maximum conventional cathode-follower action, the potential of cathode 1| is somewhat in excess of the potential on control electrode 10 and that across sweep capacitor 64. If it were not for the biasing effect of the capacitor operation of the circuit, but in one tested embodiment it was found to be useful in reducing a small voltage impulse which might otherwise occur at cathode 1| of tube 14 during the recharging period of capacitor 62.

The sequence of operations which has been described is repeated at the time of each negative voltage impulse on terminal 28, until the voltage appearing at the cathode 1| reaches the predetermined maximum Voltage value. Coincident with this condition, the relativelylongpositivepotential reset impulse 3| is received at terminal 86 from pulse source I6 (Fig. l), which pulse acts both to reset the stair-step wave generator |8 by rapidly discharging sweep capacitor 614, and to disable the pulse repeating portion of the circuit comprising tube 24. The pulse repeating portion of the circuit is disabled by the coupling of this positive voltage impulse 3| through capacitor 96 and resistor 39 to control electrode 31 of tube 24. This action increases the potential of .control electrode 31 to the point of anode current saturation, and reduces the potential of control electrode 48 and that of cathode 51 of tube 50 to their minimum values, in the previously described manner. Thus, although stepping impulses 30 may be received on control electrode 26 during the interval vof pulse 3|, the potential of control electrode 48 is lowered to a value where these pulses are ineffective in controlling the po tential of cathode 51. The discharge of sweep capacitor 64 is effective through tube 94 and the right half of tube 14. When the positive pulse appears at terminal 86, the normally non-conducting left section of tube 94 is made conductive, thereby cutting oi the normally conducting right half which includes anode 95. The` potential rise at anode 95 is sufficient to raise the potential of control electrode 13 of tube 14 positive to a point at which this electrode draws current, thereby reducing the plate resistance of the right half of tube 14 to its minimum value, and

holds this tube at this minimum value until after sweep condenser 64, which is connected over conductor 59 to anode 12 of tube 14, has been discharged to substantially the potential of cathode 15. It will thus be seen that the action of tube 94 is to cause the amount of grid current which iiows in control electrode 13 of tube 14 to be accurately SEI-resistor 82 combination, this increased potential would cause charging current to flow through the right and left paths of tube 66 to further increase the charge of sweep capacitor 64. This action would be regenerative, and sweep capacitor 64 would gradually acquire an unwanted charge. This effect is prevented by the blocking bias that is developed across capacitor 80 when capacitor 62 is discharged through resistor 82. The charging current which flows through the'` right path of tube 66 creates a potential drop across this combination in such manner that the anode 84 of tube 66 is suitably negative with respect to the associated cathode.

Thus, in the equilibrium state between `voltage impulses on terminal 23, conduction through the 'double diode 66 is blocked, since the potential of anode 84 is suitably negative with respect to the potential of cathode 68, with which it is in series connection. Resistance 8| is not essential vto the 4 controlled in order that this current may be taken into account in the design of the dividing network 60, 19.

At the termination of the positive voltage impulse 3|, the right half of tube 24 is restored to non-conduction. The left half of this tube is restored to its conducting state, and this portion of the circuit is reconditioned for the reception of the negative voltage impulse 36 from terminal 28. In addition, anode current saturation is restored in the right half of the tube 94; and the right half of tube 14, including anode 12 and cathode 15, is returned to non-conduction, thus readying the circuit for another incremental potential change or build-up across the sweep capacitor 64.

As the potential on capacitor 64, and at cathode 1| of tube .14, changes, or steps from one value to another, the potential of control electrode |56 of tube |54 in relaxation oscillator I4 (Fig. 3) is progressively increased. 'I'hese potential changes are reproduced across the grid return resistors |3|, |32 and potentiometer |33, thereby controlling the oscillation frequency of oscillator I4. In the conventional manner of relaxation, or multivibrator oscillators, only one of a'coaeso l 1 tubes V|86, |08 is conductive at any given instant. The voltage at the mid-point of potentiometer |33 is positive to avdegree that both of the tubes |06, |08 may possibly be conductive at the start of operations. To overcome this, key |62 is provided, vthe vtransfer Ycontact of which is connected to the vcommon point of Va capacitor-resistor Ycomsuch las to .starttubes |06 and |08 oscillating in vthe desired manner. `If it is 'assumed that tube n|06 iirst regains conduction, the voltage drop across its nanode load resistor |25 is coupled directly through the 'tube 35 'and coupling 'capacitor |41 to control-electrode |30oftube -v| 08. This 'change iof potential Aacross capacitor "|41 drives control electrode |30 negative in the usual manner. Capacitor '14| now'charges through -resistor l|32 toward `the lpotential :that exists at the midpointof potentiometer |33, which potential is a "function'o'fthe potential of thecontrolelectrode ofjtube 156. When capacitor |4| Vacquires Asuicient charge fto' raise control ielectrode |30 to a -point Ywhere current'flows in tube |08, the Jpotenltials Nof anode |20 of tube '|08 and of control electrode V|36 and cathode |46 of tube *|31 are changed inthe negative direction, which 'change vis `coupled through lcapacitor |43 =to the control `electrode -|728 -of tube |06, fto renderthis latter tube cut-olf `ahnen-conducting. `4'Ihe-voltage'rise at anode ||8-of tube "|06 is reflected in the control grid-cathode circuit of tube |35, and is control-electrode 1300i tube '|08.

Capictor -|'4| now charges from the source of anode supply through the principal charging .pathcomprislng thef-space discharge path of tube `-|i35,}capacitor IH, varistor `'|24 and theground 'return to the negative -terminal Aof the supply source. -A second, and inconsequential, charging Ypath isi momentarily provided over the path-com- :prisingthespace discharge pathofvoltage reguode lpathiof tube |35, .capacitor '|4|,varistor :|24 and the ground return to the supplysource. The

. currentow1in-this second path 'is-additive to the V"current ow zin the primary path, and forms-only -arelatively small part ofthe -totalchargingfcurrent; It should'fbevnotedthat neither'anode'load yresistor |25 Snor 26 -is included -in 'the principal charging :path for capacitor =`|f4|. 4The same is 'true of 1fthe :charging lpath ffor vcapacitor |43.

d'Both ofthese capacitors are, therefore,-charged,

either :positively or negatively, primarily through the relatively -lowiimpedance 'space discharge path -Inthe type of multivibratoroscillator wherein the conventional capacitive coupling is used'fbetween -thecoupled anode and grid electrodes, it

is `Well recognized that-the valuefof the grid-re- 'capaciton Yare largely determinative ofthe maximum-'oscillation frequency, and, hen-ce, the degrees of `stability Aand linearity that may 1be "ex- L pected `in the oscillator at va specified loperating frequency, or frequencies. Since, in theoscillator turn-resistor must `be `greater than that of 4the 'anode resistorin a`-fairlyjwell deiinedratiq :These values, together Vwith the 'value -of the coupling of tube V'|12 also deviates, and thepotential at the v of this yinvention the anode resistor Iremoved Vfrom the frequency determi-native branch-of the circuit, it follows that it may `have its optimum value Without regard to the grid resistor. Similarly, since-a low impedancespace discharge path is substituted for the anode resistor in the `ca- '-pacitor charging circuits, much higher oscillating frequencies may be realized with greatly `i-ncreased stability of operation `and linearity `of frequency change.

Since the coupling capacitors |'4|, '|43 -are charged from a low impedancesource 'and'acquire their `charge in a shorter time than is the case in the conventional type of multivibrator oscillator, the `potential ofthe 'control electrode A|28 or |-30 is raised more Apositively than is usual. This increased grid potential vresults in greatly lincreased screen current flow, and if no precautionarym'easure were taken it would adversely affect the linearity of the change of frequency asthe potential at the 4mid-point of `potentiometer |33 is changed, Iand would also be "detrimental'to the tube. To obviate this conditiorrthe unidirectiona1 conducting devices |22, |24, which may suitably be germanium varistors of .the AWell-'known type, are connected between the control electrode andthe cathode Vof the respective tube. "Therefore, When the highly' positive impulse is coupled `through vcoupling capacitor 14|, .or |43, this impulse is shunted to ground at the .cathode rather .thanbeing permitted to increase 4theipotential of the respective control `electrode vwith the ant undesirable results.

Output impulses 'from oscillator |4 are derived across resistor l|48 in the cathode circuitfof tube attend- |'3'|, and are coupled through capacitor 3|'50 yand vconductor |`5| 'to a suitable utilization circuit which, as was previously stated, maybe Ia balanced modulator 1|`2 (Figli). A second Aoutputis taken from the oscillator 'through .the ,connection of coupling capacitor |40 to vcathode |38 `of 'tube |35. This output is Vconnected over .conductor |42 tothe frequency-stabilizing circuit-of Fig. l4.

'Proceeding now to the .operation .of Ythe .fre- Y V.the .minimum Aoperating frequency :of .oscillator I4. Therefore, :as the :frequency of oscillator |4 varies from its minimum value, the amplitude of the alternating component appearingatfanode v|'|-| of tube ,|44 alsochanges. This change-riscopposite .to the frequency changeVthatitsfwith frequency increase the amplitude of the alternating component decreases. As was previouslystated, the ,control grid bias derived from potentiometerl |84 is chosen f-such that -cathode L| 81 resides .at za Ypredetermined .suitable voltage,` for rexample, V`ten .to Ytwentyvolts, lpositive with respect to ground. .when its .control electrode excited by oscillations that are derived when oscillator |4is-op erated at its desired minimum frequency. As the frequency .of oscillator `I4 .deviates from itsloptimum values, the magnitude-of the .alternating components that Vappear von' control electrode 1| 83 cathode ends of resistors |i88.and '|92 rises above or jfalls ibelow the optimum value iin reverse rela- :tion with variationsinthe 'frequency of oscillator I4. During intervals when relay |94 is operated, the potential across cathode resistor |92 is applied through resistor |99 to storage capacitor |96 in the control grid circuit of tube |14, and is isolated on this capacitor when relay |94 is released. The potential across storage capacitor |96 is repeated across cathode load resistor |15, and is connected over contact of key IBI and conductor |19 to the lower terminal of resistor |68 in the voltage combining resistance network in the control grid circuit of tube |54 of oscillator I4. The changes in this fed back voltage are such las tend to return oscillator I4 to its desired minimum frequency.

Since the frequency of oscillator I4 is being successively stepped between predetermined minimum and maximum values by the potential changes at potentiometer |64, caused by the stairstep output wave from generator I8 (Fig. 2), it

is apparent that the frequency indicating voltageacross cathode resistor |92 (Fig. 4) is constantly varying. If this changing voltage were applied to storage capacitor |96, the frequency of oscillation of oscillator I4 would be accordingly varied by this feedback action. This condition is avoided by interconnecting cathode resistor I 92 and storlage capacitor |96 only during the relatively short interval when capacitor 64 (Fig. 2) has its minimum potential Value, and oscillator I4 (Fig. 3) has its minimum operating value. To this end, the normally open contacts of relay |94 are in series with the connection between resistor I 92 and capacitor |96.

It was previously stated that the positive voltage pulse 3| on terminal 86 eifectuated the discharge of storage capacitor 64 and incapacitated, during its period, the pulse repeating function of tube 24 in generator I8. In addition to these functions, the impulse 3| indirectly controls the operation of relay |94. Conduction is started in the left half of the single-trip multivibrator comprising the double triode |16 concurrently with pulse 3 I. This condition will prevail for a period that is determined by the time constant of resistor 294 and capacitor 2 I8, after which conduction will return to the right half of the multivibrator, and a positive voltage impulse Will appear at the anode 2I6 of tube |16. The duration of the cut-off period of the right side of this multivibrator is chosen such that oscillator I4 may be returned from its maximum to its minimum frequency Value, and all transients may have subsided, before the storage capacitor I 96 is connected to cathode load resistor I 92. In one tested embodiment, this time was suitably about seven milliseconds when the duration of the positive pulse on terminal 86 was about twenty-two milliseconds. When the potential of anode 2I6 is increased at the end of its conduction interval, a positive voltage pulse is transmitted through coupling capacitor 229, which pulse forces con- .duction in the left half of the single-trip multivibrator comprising tube |18. Conduction in the left half of |18 cuts off the right half of this tube, and the resulting increased potential at anode 224 of tube |18 is sufficient to cause triode 228 to operate at anode current saturation; and, since the Winding of relay |94 is included in its anode circuit, to operate this relay and thereby complete the circuit between cathode resistor |92 and storage capacitor |96. At the end of the cut-olf period of the right half of triode |18, conduction in triode 228 is interrupted, and the contacts of relay |94 are opened. In this manner, then, at a suitable time, for example, seven milliseconds,

CII

rafter the appearance of the positive voltage impulse on terminal 86 the contacts of relay |94 are closed and the potential of storage capacitor |96 is regulated in accordance with the operating frequency of oscillator I4 in such manner that this oscillator tends to be returned to its optimum minimum value. This readjusted biasing voltage prevails throughout the following frequency sweep, after which the frequency-checking and bias adjusting process is repeated during the next minimum frequency operating interval.

Although this invention has been described as being incorporated in a specific apparatus, in which various of the circuits parameters have been. specifically designated by Way of illustrative example, it should be appreciated that the invention is not limited to the stated values nor to the specific structure described herein. Various modications which do not depart from the spirit and scope of the invention will suggest themselves to those skilled in the art to which this invention pertains.

VCertain of the subject-matter of this disclosure is the basis of a divisional application of the present inventor, Serial No. 243,958, filed August 28, 1951, for Variable-Frequency Relaxation Oscillator.

What is claimed is:

l. A relaxation oscillator comprising a plurality of vacuum tubes each of which has at least a cathode, an anode and a control electrode, anodecathode and control electrode-cathode circuits therefor, a source of potential the positive terminal of which is resistively connected to the anodes of a rst and second of said tubes and which is directly connected to the anodes of a third and fourth of said tubes, the negative terminal of said source being directly connected to the cathodes of said first and second tubes and being resistively connected to the cathodes of said third and fourth tubes, conductive connections between the control electrode and cathode of said rst and second tubes, conductive connections between the anode and control electrodes of said first and third tubes and said second and fourth tubes, respectively, and capacitive couplings between the control electrode and cathode of said second and third tubes and said first and fourth tubes, respectively.

2. A relaxation oscillator comprising a pair of oscillator tubes and a pair of amplifier tubes, each of said tubes including at least an anode, a cathode, and a control electrode, a source of potential having a positive terminal resistively connected to the anodes of said oscillator tubes and directly connected to the anodes of said amplifier tubes and having a negative terminal directly connected to the cathodes of said oscillator tubes and resistively connected to the cathodes of said amplifier tubes, a source of positive potential and resistive connections between said source and the control electrodes of said oscillator tubes, means interconnecting the anode of each oscillator tube With the control electrode of a respective amplifier tube, and means interconnecting the cathode of each amplifier tube and the control electrode of a respective oscillator tube.

,3. The oscillator of claim 2 in which the interconnection between theanode of each oscillator tube and the control electrode of the respective amplier tube is a direct conductive connection.

4. The oscillator of claim 2 in which each means interconnecting the cathode of each amplifier tube and the control electrode of a respective oscillator tube is a capacitive coupling circuit.

.15. The oscillator ofclaim -2 in .which said source 4of positive .potential is a cathode-coupled amplifier, the cathode circuit of which includes 'the resistive connections to the control electrodes 4of .said oscillator tubes.

.6. 4A relaxationoscillator comprising a .pair of v.oscillator tubesand a pair 'of amplifier tubes,ieach of .said .tubesincluding at leastan anode, a catliodeiandaycontrol electrode, .a source of potential having apositive terminal conductivelyconnected to the anode of .each tube, the connections to ysaid soscillatortubes :including individual resistors, and ihaving .a negative terminal conductively 'connectedvto the .cathode vof zeach tube, the cathode connections tosaid amplier ,tubes.includingin ldividual resistors, .the .control electrode of each oscillator tube .being resistively coupled rto a source of variable Vpositive potential, :a .unidirecftional .conducting device connected between the Acontrol electrode and cathode of each :oscillator tube, a `conductive coupling .between the .anode of each oscillator tube and the control electrode Vof a conjugate respective amplifier tube, and a vcapacitive :connection .between the cathode `of o said conjugate amplier tube and the control elecxtrodesof Athe .other of said oscillator tubes.

7. The oscillator of claim 6 in which Veach of said unidirectional conducting devices is poled Vsuch that it readily conducts electrons fromsaid cathode to said control electrode.

8. The oscillator of ,claim '6 in ywhich keach of .said unidirectional,conducting devices is a germani'um varistor.

.9. .The ,oscillator of claim 6 in Which saidsource of variable positive potential comprises a cathode- .coupled amplier, the cathode circuit of which includes the resistive connections to the .control .electrodes .ofsaid oscillator tubes.

MILTON E. MOHR.

REFERENCES CITIED The following references are -of record in the le of this patent:

UNITED VSTATES PATENTS skeuett Manel., 1950 

